Energy issues are key issues which concern the sustainable development of China, and it is an important national policy of the country currently to seek and develop alternative secondary energies. Lithium ion batteries are high-energy batteries which have been developed rapidly in the past decade, and now have already become an emphasis direction of the new energy industry development of China as they have a high voltage, a high specific energy, a long cycle period, low environmental pollution and other advantages. A positive electrode material is an important constituent part of a lithium ion battery, and is also a part with the highest proportion of the cost in a lithium ion battery.
As a new lithium ion battery positive electrode material, LiNi1/3Co1/3Mn1/3O2 has overall electrochemical properties superior to single-component oxides, such as, LiCoO2, LiNiO2 and LiMnO2 due to the synergistic effect of nickel, cobalt and manganese. In addition, since LiNi1/3Co1/3Mn1/3O2 has a stable structure and good thermal stability, and has low costs and low toxicity compared with LiCoO2, the ternary material is a lithium ion battery positive electrode material which is considered to be capable of widely replacing LiCoO2 to be applied to small-sized lithium ion batteries, and is very likely to be applied to an EV and a HEV as a power battery material, and the material has become a hot spot of lithium ion battery researches around the world.
Contents of the Invention
Aiming at the need of improving the properties of existing ternary positive electrode materials for lithium ion batteries, an object of the present invention is to provide a modified ternary material and a precursor thereof, achieving the effective enhancement of the cycle stability, thermal stability and compacted density of a ternary material by adjusting the components and upgrading the production process technologies, on the basis of not increasing the production and material costs.
Another object of the present invention is to provide a process for preparing the modified ternary material and the precursor thereof mentioned above; the preparation technology for the precursor of a lithium ion battery positive electrode material provided in the present invention changes the prior preparation process with a single salt solution, and increases the ratio of the cobalt element in the beginning stage, enabling the microscopic structure inside the material to be more compact, while the material grows following the original crystal structure in the course of gradually changing the salt solution components, increasing the compactness of the material; and at the same time, the migration rate of lithium ions inside the material is increased. As external nickel and manganese elements increase, partial Ni0.5Mn0.5 structure is formed, effectively enhancing the cycle stability and thermal stability of the modified ternary material.
In order to solve the technical problems mentioned above, the technical solutions involved in the present invention are as follows:
A precursor of a modified ternary material for a lithium ion battery positive electrode material has a composition of the following molecular formula: Ni1/3Co1/3Mn1/3(OH)2; and consists of three layers, wherein an inner layer of the precursor is a ternary material with a cobalt content of greater than ⅓ and identical nickel and manganese contents, and the molecular formula of said inner layer of the precursor is: (Ni1/3−xCo1/3+2xMn1/3−x)(OH)2, where 0<x≦⅓; an outer layer of the precursor is a ternary material with a cobalt content of 0 to ⅓ and equal nickel and manganese contents, and the molecular formula of said outer layer of the precursor is: (Ni0.5−yCo2yMn0.5−y)(OH)2, where 0≦y<⅙; and an intermediate layer of the precursor is a concentration-gradient composite material of the above two materials of the inner layer and the outer layer of the precursor.
A process for preparing the precursor of a modified ternary material for a lithium ion battery positive electrode material of the present invention has the particular steps of:
(1) adding a ternary salt solution A of nickel, cobalt and manganese into a reaction kettle at a certain rate, wherein the molar ratio of Ni:Co:Mn=(⅓−x):(⅓+2x):(⅓−x), where 0<x≦⅓), carrying out a coprecipitation reaction with an alkali solution to obtain a solid-liquid mixture, the molecular formula of the precipitated solid being (Ni1/3+xCo1/3+2xMn1/3−x)(OH)2, where 0<x≦⅓, so as to form an inner layer part of the precursor;
(2) in the following course of injecting the ternary salt solution A of nickel, cobalt and manganese and a ternary salt solution B of nickel, cobalt and manganese, adjusting the flow rate of the alkali solution at any time to keep the pH value of the solution in the reaction kettle between 10-12;
first, continuing the addition of the ternary salt solution A of nickel, cobalt and manganese into the reaction kettle at a decreasing rate with a decrement of 100-1,000 ml per hour and at the same time, gradually adding the ternary salt solution B of nickel, cobalt and manganese with the same total molar ratio concentration into the reaction kettle at an increasing rate with an increment of 100-1,000 ml per hour from zero, wherein the molar ratio of Ni:Co:Mn=(0.5−y):2y:(0.5−y), where 0≦y<⅙; so as to form an intermediate layer part of the above precursor which connects the inner layer and the outer layer and has a concentration gradient in the precursor;
(3) when the injection speed of the ternary salt solution A of nickel, cobalt and manganese has decreased to zero, continuing the injection of the solution B until the complete injection into the reaction kettle with a constant speed at a certain rate, so as to form an outer layer of the precursor coated outside of the intermediate layer part of the precursor mentioned above; and
(4) separating the solid-liquid mixture after the reaction in step (3) is completed by means of centrifugal filtration, washing the same to be neutral, and oven-drying the same at 60° C.-200° C. for 4-10 h; the general molecular formula of the precipitated solid obtained being (Ni1/3Co1/3Mn1/3)(OH)2, and the precipitated solid being the precursor of a modified ternary material.
On the one hand, said ternary salt solution A of nickel, cobalt and manganese and said ternary salt solution B of nickel, cobalt and manganese have the same total molar ratio concentration, and the volume ratio of the two injected into the reaction kettle is 1 to 10.
A modified ternary material for a lithium ion battery positive electrode material of the present invention has the precursor mentioned above.
A process for preparing a modified ternary material for a lithium ion battery positive electrode material of the present invention is preparing the product by crushing the precursor mentioned above, mixing the same with a lithium source and calcining, that is to say, mixing the powder of said precursor with a lithium source and calcining at 300° C.-1,200° C. for 8-30 h to form a modified ternary material.
Further, the process is as follows: first, obtaining a precursor of a modified ternary material with the general molecular formula of the precipitated solid of (Ni1/3Co1/3Mn1/3)(OH)2 according to the preparation process for a precursor of claim 2; and then, after mixing well the above precursor with a lithium source at a molar ratio of 1:1 to 1:1.2, subjecting the same to multi-stage calcination in a muffle furnace, with the calcination temperature of 300° C.-1,200° C. and the calcination time of 8-30 h, and after the multi-stage calcination, cooling, crushing and sieving to obtain a modified ternary material.
Compared with the prior art, the beneficial effects of the present invention are as follows:
For the modified ternary material and the precursor thereof provided in the present invention, on the premise of not increasing the preparation cost of a ternary material, according to the combinations of different composition proportions and different volume amounts used of the ternary salt solution A of nickel, cobalt and manganese and the ternary salt solution B of nickel, cobalt and manganese, a modified LiNi1/3Co1/3Mn1/3O2 the precursor of which has different internal structure is obtained. Compared with the LiNi1/3Co1/3Mn1/3O2 material with a homogeneous internal structure, the series of the modified ternary materials have a similar discharge specific capacity, a higher tap density, and better cycle stability and safety performance, and the rate performance of some materials is also enhanced relatively largely, having an significant cost-performance advantage, and being more suitable for the application on a power battery.